The present invention relates to a method and apparatus for the recovery of heat and easily evaporable components, such as methanol and turpentine, from hot gases, especially from the expansion vapors of waste liquor. This invention relates especially to a method and apparatus for the fractionation of the condensates in connection with the evaporation of liquids, and it is intended mainly for the pre-evaporation of a waste liquor, such as sulfate black liquor, emerging from a continuous-working digester, whereby the black liquor is concentrated from approx. 15-18% to approx. 23-25% in film-evaporation devices working according to the falling film principle, by using for the evaporation the so-called digestion buffer vapor.
In the continuous-working cellulose digester currently in use, the black liquor passing into the evaporator is taken out at a minimum absolute pressure of approx. 8 atm. and at a temperature of approx. 170.degree. C. Thus it contains a considerable amount of thermal energy, which can be utilized in the process.
So far the black liquor has usually been pre-evaporated first by lowering the pressure during two successive expansion stages so that a liquor vapor at approx. 120.degree. C., suitable for the expansion of digestion chips, is obtained from the first stage, and the vapor at approx. 100.degree. C. generated during the second stage is used for heating water. At this time the temperature of the liquor is approx. 100.degree. C., which is regarded as a suitable inlet temperature for the final evaporation. The final evaporation is performed in a multi-stage evaporator based on the indirect transfer of heat; in this case, fresh vapor which yields the thermal energy required by the process is fed to the first stage. Such a use of heat is not as economic as it could be.
If the digester expansion vapor is used as one source of heat for the black-liquor evaporator, the need for fresh vapor decreases. Advantages are also gained in terms of environmental protection.
The use of black-liquor expansion vapor for the pre-evaporation in the buffer evaporator is known per se from, for example, U.S. Pat. No. 3,286,763, and an evaporator suitable for this purpose has been introduced in patent application Ser. No. 644,714; this contains laminae inside which heating vapor is fed and which serve as heat exchangers. The liquid to be evaporated is caused to flow onto the outer surfaces of the laminae, where it flows downwards. The direction of the vapor flow is also downwards and the produced condensate is removed at the lower part of the apparatus. One object of this invention is to make the fractionation of the condensates of such a buffer evaporator more effective.
In the evaporation of black liquor the easily evaporable components are removed along with the vapor flow during the first evaporating stages. If a pre-evaporator, for example, a buffer evaporator, is available, a large proportion of these easily evaporable components is separated from the black liquor in the pre-evaporator and is condensed together with the outlet vapor. The evaporating components in the evaporator thus pass from the liquor into the condensate. Because the quantities of these components, especially of methanol and turpentine, are large and for environmental reasons must be removed from the condensate before it is released into the watercourse, they must be separated from the condensate water. This separation requires a stripping column into which fresh vapor is fed.
The object of the present invention is to provide a more economic method and apparatus than previous ones for the recovery of heat and easily evaporable components from hot gases and vapors.